1. Field of the Invention
The present invention relates to a method and apparatus for continuously monitoring cardiac output in which accuracy is improved by improving the accuracies of the measurements of the components of the cardiac output derivation.
2. Description of the Prior Art
The heart is a major organ of a person. This pumping station circulates blood through the entire body sending critical nutrients and oxygen to maintain the body's metabolic demands. A measure of the heart's performance is therefore very important to physicians. One very useful parameter is the cardiac output (CO).
Cardiac output is the volume of blood pumped out of the heart through the aorta to the rest of the body in one minute. A "typical" 68.7 kg person with a total blood volume of 5200 ml has a CO of 5345 ml/min at a heart rate of 80 beats/min. This works out to be 67.2 ml/beat.
At present several techniques are used to measure CO. There exist both direct and indirect methods. Direct methods of monitoring CO include the Indicator-Dilution method. The Indicator Dilution method covers a range of techniques such as: thermodilution, Fick and dye dilution. These techniques generally employ the introduction of a detectable indicator into a vessel upstream and measuring the indicator concentration (time curve) downstream. The thermodilution method is used as the standard in clinical medicine for measuring CO. However, these methods are cumbersome and are only able to give average CO values. Furthermore, continuous monitoring of CO using these methods is not possible as the indicator cannot be indefinitely introduced into the patient. There is a need for on-line continuous monitoring of CO for cardiac pacing and bedside monitoring. The indirect methods employ ultrasound and electromagnetic techniques. These methods are able to provide instantaneous single stroke information. However, they are only able to give flow velocity and not volumetric flow.
Kubicek et al. introduced another non-invasive impedance technique for obtaining continuous CO in their article "Development and Evaluation of an Impedance Cardiac Output System", Aerospace Med., 37, p. 1208, 1966. This thoracic impedance method involved the use of four electrodes placed transversely around a person, two around the neck and two around the thorax. The system has two outer current injecting electrodes and two inner potential electrodes. However, the method does not yield good estimates of CO in sick patients. Nevertheless, the impedance technique provides a means of obtaining on-line continuous CO.
Another technique involving impedance measurements for the determination of CO is the measurement of ventricular volume by means of an intracardiac conductance catheter as disclosed Dimensions Obtained With Impedance Catheter," Cardiovase Res., Vol. 15, pp, 328-34, 1991. This technique measures the changing ventricular volume which is the stroke volume (CO is the total stroke volume of the heart in one minute). This method did look promising. However, it was found that the conductance is not a linear function of the volume. This could be due to the ventricle's highly irregular shape and this shape varies over the cardiac cycle.
An alternative method was proposed for a continuous cardiac output monitoring system. This method calculates CO by taking the product of the velocity of blood flow and the cross-sectional area of the artery (likely the aorta) as illustrated in FIG. 13.
A dual fiber laser doppler anemometer developed by Tjin for accurate in vivo blood low measurements can be used to measure the velocity of blood flow while the cross-sectional area of the artery can be determined by an electrical impedance method. This impedance technique is based on the relationship: ##EQU1##
where,
R is Resistance (Impedance); PA1 .rho. is the resistivity of the medium; PA1 l is the separation of the measuring electrodes; and PA1 A is the cross-sectional area.
In this relationship, it is assumed that the electric field generated by the electrodes is uniform.
In 1995, Chew, H. L., et al. measured the impedance of blood using two electrodes and disclosed their method and results in "Continuous Cardiac Output Monitoring by Impedance Measurements in the Artery," FYP Report, Nanyang Technological University, p. 12, 1995. However, in a two electrode system, the electric field generated is non-uniform, which deviates from the assumption of a uniform field between the two electrodes in the above equation. This non-uniform field produces a non-linear relationship between area and resistance.
With a four-electrode probe, where the current is injected by two outer field electrodes and the potential is measured by two inner measuring electrodes, polarization problems in a non-uniform electric field are minimized.
A four-electrode system does, however, suffer from inaccuracies due to the fact that a layer of electric charge will build up at the electrode electrolyte interface. Accordingly, the measured impedance will be too high due to the extraneous charge which gives rise to additional impedance between the two measuring electrodes.
In addition, it has been found that the resistivity .rho. of the medium, typically blood, changes with temperature by about 2.4% per degree celsius temperature change. Accordingly, the cross-sectional area derived from the resistivity of the medium can vary with temperature in a manner which affects the indicated CO.